Chronic low back pain may be caused by many different sources. It may start from diseases, injuries or stresses to many different structures including bones, muscles, ligaments, joints, nerves or the spinal cord. The affected structure will send a signal through nerve endings, up the spinal cord and into the brain where it registers as pain. Many different theories try to explain chronic pain. The exact mechanism is not completely understood. In general, it is believed that the nerve pathways that carry the pain signals from the nerve endings through the spinal cord and to the brain may become sensitized. Sensitization of these pathways may increase the perceived pain out of proportion to the source of the pain. Lower back pain can have causes that aren't due to underlying disease. Examples include overuse such as working out or lifting too much, prolonged sitting and laying down, sleeping in an uncomfortable position, wearing a poorly fitting backpack. Contributors to the ongoing epidemic of prescription opioid abuse, addiction, and death include opioid tolerance, withdrawal symptoms, and possibly opioid-induced hyperalgesia (OIH).